1. Field of the Invention
This invention relates to materials in which single wall magnetic domains can exist and, more particularly, to a magnetic domain material suitable for the selective generation of normal, and not hard, single wall magnetic domains.
2. Brief Description of the Prior Art
It is well known in the art to use magnetic materials such as garnets and orthoferrites with intrinsic and/or induced (by shape, stress or growth) uniaxial anisotropy to generate single wall magnetic or bubble domains. Typically, the bubble domains are generated by applying a suitable bias field perpendicular to a sheet or layer of magnetic bubble domain material. The normal bubble domains that are induced in such a material exist over a narrow range of bias field values, typically about 15 Oersteds, and propagate in the direction of an applied bias field gradient. However, in garnet materials, bubble domains may be formed that exist over a range of bias field values of as much as approximately 40 Oersteds. In addition, these unusual bubble domains, termed hard bubbles, have low mobilities and propagate at an angle to the applied bias field gradient. Because of such properties, the presence of hard bubbles may render the garnet material unsuitable for use in bubble domain circuits and devices.
Several techniques are available for suppressing the formation of hard bubble domains. A double layer technique (Type I) is described in an article by A. H. Bobeck et al., published in the Bell System Technical Journal, VOl. 51, pgs. 1431-35, July-August, 1972. In this technique, a garnet layer (suppression layer) of low magnetic moment is interposed between a garnet bubble domain layer and a substrate. The application of a suitable bias field to form bubble domains in the bubble layer saturates the suppression layer, precluding the formation of bubble domains therein and magnetizing the entire suppression layer antiparallel to the bubble domains. As a result of the antiparallel directions of magnetization, domain walls are formed between the intermediate layer and the bubble domains, "capping" the domains. These extra domain walls, termed 180.degree. walls or caps because of the antiparallel magnetization, apparently suppress the formation of hard bubbles in the bubble layer by limiting the degrees of freedom available to the domain wall geometry. The usefulness of the Type I double layer suppression technique is limited by (1) the propensity of the suppressed bubble layer to spontaneously generate unwanted bubbles and (2) the tendency of domains to split or segment when they are stretched for the purpose of detection.
Another double layer suppression technique (Type II) is described in the paper by A. H. Bobeck et al., supra. This technique utilizes a garnet bubble domain layer having a magnetization compensation temperature below room temperature. A garnet layer which is interposed between the bubble layer and a supporting substrate possesses a lower moment than the bubble layer and has a compensation temperature which is above room temperature. Upon application of an external bias field to form bubble domains in the bubble domain layer and to saturate the interposed layer, the d-site Fe sublattices of the interposed layer and the non-bubble regions of the bubble domain layer are magnetized in antiparallel directions. This creates interfacial domain walls external to the bubble domains. That is, domain walls are created at the interface of the two layers between, but not along, the lower ends of the bubble domains. The authors report that hard bubbles are eliminated by such a domain wall. However, the operability of this arrangement is obviously limited to a narrow temperature range and may be temperature sensitive within this range.
A single-layer hard bubble suppression technique that utilizes ion implantation to form a wall or boundary in the upper surface of a magnetostrictive garnet bubble domain layer is described by R. Wolf and J. C. North in the Bell System Technical Journal, VOl. 51, pgs. 1436-1440, July-August, 1972. The ion implantation is accomplished in a thin region in the upper surface of the garnet layer. The constraints exerted by the rest of the layer on the implanted region create a new moment of magnetization parallel to the surface and perpendicular to the direction of magnetization of the bubble domains. The magnetization of the implanted region apparently creates an extra domain wall, a 90.degree. cap, in bubble domains induced in the unimplanted region of the layer, thereby eliminating hard bubble domains by decreasing the number of available degrees of freedom. However, from a practical standpoint, the ion implantation technique is limited to garnet materials having negative magnetostriction constants of relatively large absolute values. In addition, the ion implanted region physically separates the generation and other device structures from the bubble domain layer and presumably renders bubble devices formed therefrom less flexible in design.
Another hard bubble suppression technique, also a 90.degree. capping technique, is disclosed in copending United States patent application Ser. No. 375,999, entitled MAGNETIC BUBBLE DOMAIN COMPOSITE WITH HARD BUBBLE SUPPRESSION, by Rodney D. Henry and Paul J. Besser, filed July 2, 1973, now abandoned, and assigned to the common assignee. This 90.degree. capping technique utilizes a magnetic garnet, hard bubble suppression layer that may be (1) interposed between a bubble domain layer and a supporting substrate or (2) formed directly on the bubble domain layer, which itself is grown on the substrate. The hard bubble suppression layer has stress-induced anisotropy such that there is an easy axis of magnetization which is approximately parallel to the interfacial plane of the bubble domain and the suppression layers and perpendicular to the direction of magnetization of the bubble domains. Because the easy axis of magnetization of the suppression layer is parallel to the plane of the bubble domain layer, (90.degree. relative to the bubble domain magnetization direction), the suppression layer forms an extra domain wall or cap to the bubble domain.
Although prior art suppression techniques may be highly effective, they require additional processing steps and/or structures. As may be appreciated, it is desirable to have a hard bubble suppression technique that eliminates the cost in time and money of such additional processing.